In vehicular transport and traffic management, it is known to use Intelligent Transport System (ITS) applications for supporting drivers. In this way, traffic safety can be improved by providing the drivers with information which allows for making smarter decisions. Such ITS applications may involve transmitting information between different vehicles, e.g., in the form of a Cooperative Awareness Message (CAM). The information may be used for providing a warning or guidance to the driver, e.g., in the form of an emergency vehicle warning, an intersection collision warning, a slow vehicle warning, or a motorcycle approaching indication. The information may be transmitted using a radio technology for vehicle-to-vehicle (V2V) communication, e.g., as specified by the IEEE 802.11p standard, also referred to as WAVE (Wireless Access in Vehicular Environments). According to the IEEE 802.11p standard, a wireless ad-hoc network may be formed between different vehicles. Such ad-hoc networks are also referred to as Vehicular Ad-Hoc Network (VANET).
CAMs are messages which are typically periodically broadcast by a vehicle to inform nearby vehicles about the current status of the vehicle. CAMs may for example be used for transmitting the current geographical position, speed, and/or basic attributes of the vehicle. A vehicle may receive CAMs from other vehicles and utilize the information provided in the CAMs for supporting its operator, e.g., by providing a warning or other guidance.
More details on CAMs can be found in ETSI TS 102 637-3 “Intelligent Transport Systems (ITS); Vehicular Communications; Basic Set of Applications; Part 2: Specification of Cooperative Awareness Basic Service”. Here, a generation of cooperative applications is described, which are based on the IEEE 802.11p specifications. The traffic model for the transmission of such CAM messages is specified to use periodic broadcast traffic consisting of approx. 500 payload bytes with a repetition rate 5-10 Hz and event-driven broadcast traffic consisting of approx. 500 payload bytes. Both traffic types may exist at the same time.
The ad-hoc network according to IEEE 802.11p is a contention based system, in which different V2V communication devices compete for access to the same radio channel. Accordingly, there is also a risk of colliding access attempts by different V2V communication devices. Such collisions may in turn result in delays until a V2V communication device successfully gains access to the radio channel.
In “LTE4V2X: LTE for a Centralized VANET Organization” by G. Remy et al., IEEE Globecom Proceedings (2011), it is proposed to use infrastructure of a cellular network to organize a VANET into clusters of V2V communication devices. In this case, the cellular network is based on the LTE (Long Term Evolution) technology specified by 3GPPP (3rd Generation Partnership Project). In particular, it is described that a base station of the cellular network, referred to as eNodeB (eNB), may organize a cluster formed of a cluster head (CH) and cluster members (CMs). Over the cellular network, the eNB receives Floating Car Data (FCD) from multiple V2V communication devices and uses this information to elect the CH and associate the CMs to the cluster. The eNB then broadcasts a cluster description over the cellular network. The cluster description indicates the identity of the CH and the identities of the CMs. By receiving such cluster description, a V2V communication device is informed about the cluster to which it is assigned.
Within the cluster, the CH acts as a gateway between the CMs and the cellular network. Specifically, the CH aggregates FCD transmitted by the CMs over the VANET and sends the aggregated FCD data over the cellular network to the eNB. For the VANET communication within the cluster, Time Division Multiple Access (TDMA) is used by the CMs. Specifically, each CM uses a different time slot, derived from the order of the identities of the CMs in the cluster description.
However, the above LTE4V2X solution is not optimized for transmission of CAMs between V2V communication devices. Rather, the VANET communication mainly consists in communication of FCD from the CMs to the CH. In the case of CAMs it is in turn desired that typically all V2V communication devices within a certain communication range can receive the CAMs. This may in turn cause collisions of CAM transmissions. In the LTE4V2X solution, collisions between VANET transmissions within a cluster can be avoided. However, collisions between VANET transmissions of different clusters are not addressed. Still further, the centralized management of the clusters may cause significant load on the cellular network and requires cellular connectivity for each V2V communication device.
Accordingly, there is a need for techniques which allow for efficiently utilizing a cellular network for management of V2V communication over a VANET.